In Situ 87Sr/86Sr of Scheelite and Calcite Reveals Proximal and Distal Fluid-Rock Interaction During Orogenic W-Au Mineralization, Otago Schist, New Zealand

The ability of hydrothermal scheelite (CaWO4) and calcite (CaCO3) to bind Sr in their crystal lattices makes them useful minerals for tracing fluid-rock interaction in mineralizing systems. Strontium isotopes in scheelite, calcite, and epidote were measured in situ by laser ablation-multicollector-inductively coupled plasma-mass spectrometry to assess the extent of fluid-rock interaction for Early Cretaceous orogenic W-Au mineralization occurring in the Otago Schist in New Zealand. Scheelite Sr isotope populations, regardless of their geographical location or stage of mineralization, are mostly homogeneous and cluster between Sr-87/Sr-86 = 0.7068 and Sr-87/Sr-86 = 0.7087, with the lack of Rb indicating that these are the initial ratios of the mineralizing fluids. However, while scheelite Sr-87/Sr-86 values in the Hyde-Macraes shear zone and the Barewood and Bendigo deposits overlap with the age-corrected Sr-87/Sr-86 values of the host Rakaia terrane, scheelite Sr-87/Sr-86 values in the Glenorchy and Waipori deposits are commonly more radiogenic than the age-corrected values of the host Caples terrane rocks. Because the Caples terrane was thrust over more radiogenic rocks before mineralization, the Caples terrane scheelite deposits are interpreted to result from fluids that were derived from deep crustal dehydration reactions within the underlying radiogenic and dominantly metaturbiditic (grayschist) Rakaia and Aspiring terranes, and/or that acquired their Sr isotope signatures from extensive fluid-rock reaction with Rakaia and Aspiring terrane grayschists during ascent. It is also evident that the scheelite Sr-87/Sr-86 values in the Caples terrane become more similar to the host-rock values with increasing incorporation of wall rock at the deposit scale as well as with increasing distance above the Caples-Rakaia/Aspiring terrane boundaries. Late-stage calcite that has replaced scheelite takes on the scheelite Sr isotope composition, whereas tabular calcite grains in the same samples have less radiogenic compositions that are equivalent to the wall rock. Application of in situ Sr-87/Sr-86 analysis therefore reveals that (1) there was extensive fluid-rock exchange of Sr during the formation of orogenic W-Au deposits, and (2) the Cretaceous W-Au mineralizing fluids in the Otago Schist were mobile over at least km-scale and probably greater vertical distances.